LiDAR readout circuit

ABSTRACT

A LiDAR readout circuit is described. The readout circuit comprises an SiPM sensor for detecting photons and generating an SIPM analog output signal. A plurality of comparators are provided each having an associated threshold value and being configured to compare the SiPM analog output signal with their associated threshold value and generate a comparison signal. A time to digital converter is configured to receive the comparison signals from the plurality of comparators.

FIELD OF THE INVENTION

The invention relates to a LiDAR readout circuit. In particular but notexclusively the present disclosure relates to a LiDAR readout circuitwhich includes multiple thresholds.

BACKGROUND

A Silicon Photomultiplier (SiPM) is a photon sensitive, highperformance, solid-state sensor. It is formed of a summed array ofclosely-packed Single Photon Avalanche Photodiode (SPAD) sensors withintegrated quench resistors, resulting in a compact sensor that has highgain (˜1×10⁶), high detection efficiency (>50%) and fast timing (sub-nsrise times) all achieved at a bias voltage of ˜30V.

Traditionally LiDAR with analogue SiPMs is performed by discriminatingthe output of the SiPM against a fixed threshold corresponding toN-photons, where N is typical set to 1 to allow single-photon detection.However, in high light conditions, many close-in-time photons contributeto the output current/voltage with increments beyond the fixedsingle-photon threshold. Such contributions are therefore lost by thediscriminator liming the number of timestamps of the readout. Increasingthe threshold however results in the loss of the information of thesingle-photon events, which is important for the fast detection high andlow light levels

There is therefore a need to provide for a LiDAR readout circuit whichaddresses at least some of the drawbacks of the prior art.

SUMMARY

The present disclosure relates to a LiDAR readout circuit comprising:

-   -   an SiPM sensor for detecting photons and generating an SIPM        analog output signal;    -   a plurality of comparators each having an associated threshold        value and being configured to compare the SiPM analog output        signal with their associated threshold value and generate a        comparison signal; and    -   a time to digital converter configured to receive the comparison        signals from the plurality of comparators.

In one aspect, an amplifier is provided for amplifying the SiPM analogoutput signal in advance of the SiPM analog signal being received by thecomparators.

In a further aspect, an output of the amplifier is operably coupled toeach of the comparators.

In another aspect, a voltage divider is configured for setting therespective threshold values of the comparators.

In an exemplary aspect, the voltage divider is operably coupled betweentwo reference nodes.

In one aspect, one of the reference nodes is operably coupled to avoltage reference. Advantageously, the other one of the reference nodesis ground.

In a further aspect, a plurality of resistors are operably coupledbetween the two reference nodes.

In another aspect, the voltage divider sets a corresponding voltagethreshold level for each comparator.

In an exemplary aspect, the voltage threshold level for each comparatoris different.

In one another aspect, the voltage threshold level of two of more of thecomparators is different. Advantageously, the threshold values of therespective comparators increments sequentially from a low thresholdvalue to a high threshold value.

In another aspect, the threshold value for each comparator is determinedbased on the ambient light level.

In one aspect, a threshold determining circuit is provided.

In another aspect, the threshold determining circuit is operable to beselectively activated.

In a further aspect, the threshold determining circuit is selectivelyconnected to the LiDAR readout circuit via a switch.

In one aspect, the threshold determining circuit comprises ananalog-to-digital converter.

In a further aspect, the threshold determining circuit comprises adigital-to-analog converter (DAC) operably coupled between the ADC andat least one one of the comparators.

In an exemplary embodiment; the DAC is configured to receive a digitalsignal representative of a measured noise level output from the SiPMsensor from the ADC.

In another aspect, the DAC is further configured to receive an arbitraryvalue which together with the digital signal representative of themeasured noise level determines the threshold value for at least one ofcomparators.

In a further aspect, the SiPM sensor is located on a LiDAR device.Advantageously, the LiDAR device further comprises a laser source.

In one aspect, the laser source is configured to emit laser pulses.

In another aspect, the LiDAR device further comprises optics.

In one aspect, the SiPM sensor is a single-photon sensor.

In a further aspect, the SiPM sensor is formed of a summed array ofSingle Photon Avalanche Photodiode (SPAD) sensors.

In another aspect, the laser source is an eye-safe laser source.

In one aspect, the SiPM sensor comprises a matrix of micro-cells.

In another aspect, a digital-to-analog converter is configured forsetting the respective threshold values of the comparators.

These and other features will be better understood with reference to thefollowings Figures which are provided to assist in an understanding ofthe present teaching.

BRIEF DESCRIPTION OF THE DRAWINGS

The present teaching will now be described with reference to theaccompanying drawings in which:

FIG. 1 illustrates an exemplary structure of a silicon photomultiplier.

FIG. 2 is a schematic circuit diagram of an exemplary siliconphotomultiplier.

FIG. 3 illustrates an exemplary technique for a direct ToF ranging.

FIG. 4 illustrates an exemplary ToF ranging system.

FIG. 5 illustrates an exemplary LiDAR device.

FIG. 6 illustrates a schematic of a prior art LiDAR readout circuit.

FIG. 7 illustrates a schematic of a LiDAR readout circuit in accordancewith the present teaching.

FIG. 8A-8C illustrate LiDAR output histograms.

FIG. 9 is a flow chart illustrating exemplary steps for determining athreshold level.

FIG. 10 is another LiDAR readout circuit which includes a thresholddetermining circuit.

FIG. 11 illustrates a schematic of another LiDAR readout circuit whichis in accordance with the present teaching.

DETAILED DESCRIPTION

The present disclosure will now be described with reference to anexemplary LiDAR readout circuit. It will be understood that theexemplary LiDAR readout circuit is provided to assist in anunderstanding of the teaching and is not to be construed as limiting inany fashion. Furthermore, circuit elements or components that aredescribed with reference to any one Figure may be interchanged withthose of other Figures or other equivalent circuit elements withoutdeparting from the spirit of the present teaching. It will beappreciated that for simplicity and clarity of illustration, whereconsidered appropriate, reference numerals may be repeated among thefigures to indicate corresponding or analogous elements.

Referring initially to FIG. 1, a silicon photomultiplier 100 comprisingan array of Geiger mode photodiodes is shown. As illustrated, a quenchresistor is provided adjacent to each photodiode which may be used tolimit the avalanche current. The photodiodes are electrically connectedto common biasing and ground electrodes by aluminum or similarconductive tracking. A schematic circuit is shown in FIG. 2 for aconventional silicon photomultiplier 200 in which the anodes of an arrayof photodiodes are connected to a common ground electrode and thecathodes of the array are connected via current limiting resistors to acommon bias electrode for applying a bias voltage across the diodes.

The silicon photomultiplier 100 integrates a dense array of small,electrically and optically isolated Geigermode photodiodes 215. Eachphotodiode 215 is coupled in series to a quench resistor 220. Eachphotodiode 215 is referred to as a microcell. The number of microcellstypically number between 100 and 3000 per mm². The signals of allmicrocells are then summed to form the output of the SiPM 200. Asimplified electrical circuit is provided to illustrate the concept inFIG. 2. Each microcell detects photons identically and independently.The sum of the discharge currents from each of these individual binarydetectors combines to form a quasi-analog output, and is thus capable ofgiving information on the magnitude of an incident photon flux.

Each microcell generates a highly uniform and quantized amount of chargeevery time the microcell undergoes a Geiger breakdown. The gain of amicrocell (and hence the detector) is defined as the ratio of the outputcharge to the charge on an electron. The output charge can be calculatedfrom the over-voltage and the microcell capacitance.

$G = \frac{{C \cdot \Delta}\; V}{q}$Where:

-   -   G is the gain of the microcell;    -   C is the capacitance of the microcell;    -   ΔV is the over-voltage; and    -   q is the charge of an electron.    -   LiDAR is a ranging technique that is increasingly being employed        in applications such as mobile range finding, automotive ADAS        (Advanced Driver Assistance Systems), gesture recognition and 3D        mapping. Employing an SiPM as the photo sensor has a number of        advantages over alternative sensor technologies such as        avalanche photodiode (APD), PIN diode and photomultiplier tubes        (PMT) particularly for mobile and high volume products. The        basic components used for a direct ToF ranging system, are        illustrated in FIG. 3. In the direct ToF technique, a periodic        laser pulse 305 is directed at the target 307. The target 307        diffuses and reflects the laser photons and some of the photons        are reflected back towards the detector 315. The detector 315        converts the detected laser photons (and some detected photons        due to noise) to electrical signals that are then timestamped by        timing electronics 325.

This time of flight, t, may be used to calculate the distance, D, to thetarget from the equationD=cΔt/2,  Equation 1

-   -   where c=speed of light; and    -   Δt=time of flight.        The detector 315 must discriminate returned laser photons from        the noise (ambient light). At least one timestamp is captured        per laser pulse. This is known as a single-shot measurement. The        signal to noise ratio can be dramatically improved when the data        from many single shot measurements are combined to produce a        ranging measurement from which the timing of the detected laser        pulses can be extracted with high precision and accuracy.

Referring now to FIG. 4 which shows an exemplary SiPM sensor 400 whichcomprises an array of Single Photon Avalanche Photodiodes (SPAD)defining a sensing area 405. A lens 410 is provided for providingcorrective optics. For a given focal length f of a lens system, theangle of view θ_(x,y) of a sensor placed on the focal point and withdimensions L_(x,y) is given by:

$\begin{matrix}{\theta_{x,y} = {2 \times {{atan}\left( \frac{L_{x,y}\text{/}2}{f} \right)}}} & {{Equation}\mspace{14mu} 2}\end{matrix}$Where:

-   -   Focal length of receiver lens: f    -   Sensor horizontal and vertical length: L_(x), L_(y)    -   Sensor angle of view: θ_(x,y)

FIG. 5 illustrates an exemplary LiDAR device 600, which includes a lasersource 605 for transmitting a periodic laser pulse 607 through atransmit lens 604. A target 608 diffuses and reflects laser photons 612through a receive lens 610 and some of the photons are reflected backtowards a SiPM sensor 615. The SiPM sensor 615 converts the detectedlaser photons and some detected photons due to noise to electricalsignals that are then timestamped by timing electronics. The averagenumber of detected photons k in a typical output pulse width t iscalculated from the incident rate Q and the photon detection efficiency(PDE) as:k=ΦxPDE×T  Equation 3

Typically, the threshold for the digital readout of an SiPM is set to kto maximize the probability of detecting events. The probability ofdetecting X photon events when the average number is k is given by:

${P(X)} = {e^{- k} \times \frac{k^{X}}{X!}}$

When a single threshold, using the comparator readout circuit of FIG. 6,is set to a certain value h, the single event per pulse is registeredwith a probability given by:

${P\left( {X \geq h} \right)} = {1 - {e^{- k}{\sum\limits_{i = 0}^{\lfloor h\rfloor}\;\frac{k^{i}}{i!}}}}$

All the events occurring with probability P(X≥h′) when h′>h will not bedistinguished and therefore not counted (or timed) as separate events.

Referring to FIG. 7 which illustrates a LiDAR readout circuit 800 inaccordance with the present teaching. In regimes where the photon rateis high, having multiple threshold enables the detection of a largernumber of events. The LiDAR readout circuit 800 is configured to providea multi-threshold system by providing the output of an analog SiPMsensor 615 to a series of discriminators set at different thresholdvoltages corresponding to single, double, triple photon thresholds, etc.This multi-channel solution enables more events to be successfullydetected by the TDC 720 without the need to integrate a readout circuitinside the analog SiPM sensor. Increasing the throughput of the readoutcircuit 800 allows acquisition times to be significantly reduced whichis essential for fast sensors.

The LiDAR readout circuit 800 includes the analog SiPM sensor 615 fordetecting photons and generating an analog SIPM output signal. Aplurality of comparators 715A-715D are provided and each has anassociated threshold value and is configured to compare the analog SiPMoutput signal with their associated threshold value and generate acomparison signal indicative of the comparison. A time to digitalconverter (TDC) 720 is configured to receive the comparison signals fromthe plurality of comparators 715A-715D and time stamp the events. TheTDC 720 may be considered as a very high precision counter/timer thatcan record the time of an event to sub 1 ns resolution. The TDC may beused to measure the time of flight of a photon from a laser pulse to atarget 608 and back to the SiPM sensor 615.

An amplifier 710 is provided for amplifying the analog SiPM outputsignal in advance of the SiPM signal being received by the comparators715A-715D. The output of the amplifier 710 is operably coupled to eachof the comparators 715A-715D. A voltage divider 725 is configured forsetting the respective threshold values of the comparators 715-715D. Thevoltage divider 725 is operably coupled between two reference nodes. Oneof the reference nodes is operably coupled to a voltage referencesource. The other one of the reference nodes is ground or a node havinglower voltage level than the other reference node. A plurality ofresistors 735A-735D are operably coupled between the two referencenodes. The voltage divider 725 sets a corresponding voltage level foreach comparator 715A-715D. The voltage threshold level for eachcomparator may be different. In an exemplary embodiment, the voltagelevel of two of more of the comparators 715A-715D is different. Thethreshold value for each comparator 715A-715D is determined based on theambient light level. The threshold values of the respective comparatorsincrement sequentially from a low threshold value to a high thresholdvalue. The sequence of threshold values may include a single value,double value, triple value etc. A single value corresponds to a singlephoton level, while a double value corresponds to twice a single photonlevel, and a triple value corresponds to three times a single photonlevel.

The SiPM sensor 615 is located on a LiDAR device 600 which comprises alaser source 605. The laser source 605 is configured to emit laserpulses. Optics in the form of transmit lens 604 and receive lens 610 arealso provided on the LiDAR device 600. The SiPM sensor 605 may be asingle-photon sensor. Alternatively, the SiPM sensor 615 may be formedof a summed array of Single Photon Avalanche Photodiode (SPAD) sensors.The laser source 605 may be an eye-safe laser source. Laser sourceeye-safety limitations are detailed in standards set forth by theAmerican National Standards Institute (Ansi) Z136 series or theInternational standard IEC60825, for example Thus, it is envisaged thatthe laser source 605 is compatible with the Ansi Z136 or IEC60825standards. The average power of the laser pulses may be fixed to meeteye-safety standards set as set forth in at least one the AnsiZ136 andIEC60825 standards. It is not intended to limit the present teaching tothe exemplary eye safety standards provided which are provided by way ofexample. The SiPM sensor 615 may comprises a matrix of micro-cells asillustrated in FIG. 1.

The advantages of the LiDAR readout circuit 800 in accordance with thepresent teaching are many some of which are detailed as follows. In thescenario of high incident photon rates, having multiple thresholdseliminates the need of dynamically adjusting the threshold to avoidsaturation of the readout or to choose the best threshold in terms ofsignal to noise ratio (SNR), all the thresholds will be processed inparallel therefore building up a high SNR histogram with no need for afeedback loop which ensures faster acquisitions. For low reflectivetargets, where the low number of detected photons enlarges theacquisition time, a higher throughput allows the acquisition time to bedecreased by improving the formation of the histogram. The graph of FIG.8A is a histogram generated by the prior art LiDAR readout circuit ofFIG. 6 which has a single photon threshold comparator output. This LiDARreadout circuit operates satisfactory for low reflective targets sincethe number of returning photons (from both ambient and laser) is low.The graph of FIG. 8B is also generated using the prior art LiDAR readoutcircuit of FIG. 6 which uses a single comparator with a singlethreshold. As illustrated, it does not operate satisfactory for highreflective targets. The number of returning photons is much greater thanone potentially saturating the readout chain (shown in the histogram asa regular pattern in misleading local peaks) making the detection of thelaser peak difficult. The graph of FIG. 8C is a histogram generated bythe LiDAR readout circuit of FIG. 7 in accordance with the presentteaching. The histogram in FIG. 8C is generated using a double photonthreshold comparator output and operates much better for high reflectivetargets reducing the input event rate of the readout therefore solvingthe saturation shown by the 1-photon threshold setting. Accordingly, itwill be appreciated by those skilled in the art that by having more thana single threshold setting allows more photons to be efficiently timedand registered with less constrain on the feedback loop typicallyincluded in the readout. Ideally, with a sufficient number of thresholdsdepending on the system properties (angle of view, light conditionsetc.), the feedback can be eliminated completely by having all thenecessary threshold settings in parallel making the system faster sinceno feedback is required which is essential for fast LiDAR high framerate systems. The number of thresholds needed in a system can becalculated from the light conditions (ex. 100 klux for outdoors),maximum reflectivity, optical aperture, angle of view and sensor PDE.From the range of min-max detected photons per pulse, one can specifythe number of thresholds.

Referring to FIG. 9 which provides a flow chart 1000 illustratingexemplary steps for determining the thresholds for the respectivecomparators 715A-715D. A noise level measuring process is initiated,step 1010. The noise level is measured with no signal source (laser)activated so that the sensor is only exposed to uncorrelated light. Itsresponse is therefore a superimposition of dark noise and backgroundlight noise (ambient light) and its associated shot noise. Whenamplified, the noise coming from the amplifier is superimposed to theSiPM response. This is the input of the comparator. Therefore, this isthe voltage V_(NOISE) that must be measured to set a correct threshold,step 1020. The threshold can be set to αV_(NOISE), where α is anarbitrary value, typically >1, for noise rejection, step 1025. Once thethreshold is set, the actual measure can take place and therefore thelaser source can be activated, step 1030. The LiDAR system then operatesas previously described with reference to FIG. 8, step 1035.

Referring to FIG. 10 which illustrates an exemplary circuit 1050 whichmay be used to measure the noise level as described with the referenceto the flow chart 1000. The components which have been previouslydescribed are indicated by similar reference numerals. A thresholddetermining circuit 1052 is operably coupled between the amplifier 710and the comparator 715. The threshold determining circuit 1052 comprisesa switch 1055, an analog-to-digital converter (ADC) 1060 and adigital-to-analog converter (DAC) 1065 in the exemplary embodiment.During step 1020 when the noise level is being actively determined theswitch 1055 is closed such that the ADC 1060 is connected to the outputof the amplifier 710. The amplified signal from the SiPM 615 which isrepresentative of the noise level is relayed to the ADC 1060 and adigital value representative of the noise level is output from the ADC1060 to the DAC 1065 together with the arbitrary value α. Both arestored until the next threshold analysis is needed. While the noiselevel is being measured the laser source 605 is not activated. In step1035 the switch 1055 is opened thereby disconnecting the ADC 1060. Thelaser source 605 is activated and the amplified signal from theamplifier 710 is relayed to the comparator 715 which is set with thethreshold level as previously determined. When the LiDAR system 1050scans another point/target, a new threshold level is calculated byrepeating step 1020.

In regimes where the photon rate is high, having multiple thresholdenables the detection of a larger number of events. The LiDAR readoutcircuit 800 is configured to provide a multi-threshold system byproviding the output of an analog SiPM sensor 615 to a series ofdiscriminators set at different threshold voltages corresponding tosingle, double, triple photon thresholds, etc. This multi-channelsolution enables more events to be successfully detected by the TDC 720without the need to integrate a readout circuit inside the analog SiPMsensor. Increasing the throughput of the readout circuit 800 allowsacquisition times to be significantly reduced which is essential forfast sensors.

The LiDAR readout circuit 800 includes the analog SiPM sensor 615 fordetecting photons and generating an analog SIPM output signal. Aplurality of comparators 715A-715B are provided and each has anassociated threshold value and is configured to compare the analog SiPMoutput signal with their associated threshold value and generate acomparison signal indicative of the comparison. A time to digitalconverter (TDC) 720 is configured to receive the comparison signals fromthe plurality of comparators 715A-715D. The TDC 720 may be considered asa very high precision counter/timer that can record the time of an eventto sub 1 ns resolution. The TDC may be used to measure the time offlight of a photon from a laser pulse to a target 608 and back to theSiPM sensor 615.

This process may be repeated each time the noise coming from theenvironment changes. For example, in a LiDAR system, when the sensor ispointing at different targets, their different reflectivity determinesdifferent noise levels which must be correctly measured. It will beappreciated by those skilled in the art that the proposedmulti-threshold LiDAR system eliminates the need of the single thresholdsetting by the availability of a defined number of pre-set thresholdsvalues. The number of thresholds, and therefore comparators, is definedin the design process considering the range of incoming light levelswhich can be calculated from the optical setting of the LiDAR system(angle of view and aperture), and the min-max background light levels,according to its application.

Referring to FIG. 11 which illustrates another LiDAR readout circuit1100 which is also in accordance with the present teaching. The LiDARreadout circuit 1100 is substantially similar to LiDAR readout circuit800 and like components are indicated by similar reference numerals. Themain difference between the readout circuits is that the thresholdvalues in the LiDAR readout circuit 1100 are set using aDigital-to-Analog converter (DAC) 1105 instead of using a voltagedivide. Otherwise the operation of the LiDAR readout circuit 1100 issubstantially similar to that of LiDAR readout circuit 800.

It will be appreciated by the person of skill in the art that variousmodifications may be made to the above described embodiments withoutdeparting from the scope of the present invention. In this way it willbe understood that the teaching is to be limited only insofar as isdeemed necessary in the light of the appended claims. The termsemiconductor photomultiplier is intended to cover any solid statephotomultiplier device such as Silicon Photomultiplier [SiPM],MicroPixel Photon Counters [MPPC], MicroPixel Avalanche Photodiodes[MAPD] but not limited to.

Similarly the words comprises/comprising when used in the specificationare used to specify the presence of stated features, integers, steps orcomponents but do not preclude the presence or addition of one or moreadditional features, integers, steps, components or groups thereof.

What is claimed is:
 1. A LiDAR readout circuit comprising: an SiPMsensor for detecting photons and generating an SiPM analog outputsignal; a plurality of comparators each having an associated thresholdvalue and being configured to compare the SiPM analog output signal withtheir associated threshold value and generate a comparison signal,wherein the associated threshold values of the plurality of comparatorsare different; a time to digital converter configured to receive thecomparison signals from the plurality of comparators; and a thresholddetermining circuit coupled to the output of the SiPM sensor and to theinput of each of the plurality of comparators, wherein: the LiDARreadout circuit causes the threshold determining circuit to perform anoise measurement when a laser source is not activated to determine theassociated threshold value of each of the plurality of comparators; andthe LiDAR readout circuit subsequently activates the laser source andperforms a measurement using the associated threshold value of each ofthe plurality of comparators.
 2. The LiDAR readout circuit as claimed inclaim 1, further comprising an amplifier for amplifying the SiPM analogoutput signal in advance of the SiPM analog output signal being receivedby the comparators.
 3. The LiDAR readout circuit as claimed in claim 2,wherein an output of the amplifier is operably coupled to each of thecomparators.
 4. The LiDAR readout circuit as claimed in claim 1, furthercomprising a voltage divider configured for setting the associatedthreshold values of the comparators.
 5. The LiDAR readout circuit asclaimed in claim 4, wherein the voltage divider is operably coupledbetween two reference nodes.
 6. The LiDAR readout circuit as claimed inclaim 5, wherein one of the reference nodes is operably coupled to avoltage reference.
 7. The LiDAR readout circuit as claimed in claim 6,wherein the other one of the reference nodes is ground.
 8. The LiDARreadout circuit as claimed in claim 5, wherein a plurality of resistorsare operably coupled between the two reference nodes.
 9. The LiDARreadout circuit as claimed in claim 4, wherein the voltage divider setsa corresponding voltage level threshold for each comparator.
 10. TheLiDAR readout circuit as claimed in claim 9, wherein correspondingvoltage level thresholds for each comparator are different.
 11. TheLiDAR readout circuit as claimed in claim 9, wherein correspondingvoltage level thresholds of two of more of the plurality of comparatorsare different.
 12. The LiDAR readout circuit as claimed in claim 1,wherein the associated threshold values of the respective comparatorsincrement sequentially from a low threshold value to a high thresholdvalue.
 13. The LiDAR readout circuit as claimed in claim 1, wherein theassociated threshold value for each comparator is determined based onthe ambient light level.
 14. The LiDAR readout circuit as claimed inclaim 1, further comprising a threshold determining circuit.
 15. TheLiDAR readout circuit as claimed in claim 14, wherein the thresholddetermining circuit is operable to be selectively activated.
 16. A LiDARreadout circuit as claimed in claim 1, wherein the SiPM sensor islocated on a LiDAR device.
 17. A LiDAR readout circuit as claimed inclaim 16, wherein the LiDAR device further comprises a laser source. 18.A LiDAR readout circuit as claimed in claim 17, wherein the laser sourceis configured to emit laser pulses.
 19. A LiDAR readout circuit asclaimed in claim 16, wherein the LiDAR device further comprises optics.20. A LiDAR readout circuit as claimed in claim 1, wherein the SiPMsensor is a single-photon sensor.
 21. A LiDAR readout circuit as claimedin claim 1, wherein the SiPM sensor is formed of a summed array ofSingle Photon Avalanche Photodiode (SPAD) sensors.
 22. The LiDAR readoutcircuit as claimed in claim 17, whererin the laser source is an eye-safelaser source.
 23. The LiDAR readout circuit as claimed in claim 1,wherein the SiPM sensor comprises a matrix of micro-cells.
 24. The LiDARreadout circuit as claimed in claim 1, wherein further comprising adigital-to-analog converter configured for setting the associatedthreshold values of the comparators.
 25. A LiDAR readout circuitcomprising: an SiPM sensor for detecting photons and generating an SiPManalog output signal; a plurality of comparators each having anassociated threshold value and being configured to compare the SiPManalog output signal with their associated threshold value and generatea comparison signal, wherein the associated threshold values of theplurality of comparators are different; a time to digital converterconfigured to receive the comparison signals from the plurality ofcomparators; and a threshold determining circuit, wherein the thresholddetermining circuit is selectively connected to the LiDAR readoutcircuit via a switch.
 26. The LiDAR readout circuit as claimed in claim25, wherein the threshold determining circuit comprises an ADC(analog-to-digital converter).
 27. The LiDAR readout circuit as claimedin claim 26, wherein the threshold determining circuit comprises adigital-to-analog converter (DAC) operably coupled between the ADC andat least one of the comparators.
 28. The LiDAR readout circuit asclaimed in claim 27, wherein the DAC is configured to receive a digitalsignal representative of a measured noise level output from the SiPMsensor from the ADC.
 29. This LiDAR readout circuit as claimed in claim28, wherein the DAC is further configured to receive an arbituary valuewhich together with the digital signal representative of the measurednoise level determines the associated threshold value for at least oneof the comparators.